The object of this research is to examine the characteristics of breathing pattern adaptation in humans to changes in external mechanical load and try to separate neural and chemical influences operating in this adaptation. The specific aims of this research are: 1) to determine the time course of the ventilation and breathing pattern changes after sudden application of resistance and elastic loads; 2) to determine the influence of changes in chemical stimuli upon these adaptation dynamics; 3) to determine the influence of a background of increased stress (exercise) upon this pattern adaptation; 4) to determine the ability of the pattern modulator to handle changes in mechanical loading which occur at various points in the breathing cycle; and 5) to assess the breathing pattern adaptations with respect to considerations of optimal control using various criteria. Two kinds of loading experiments will be performed. The first (multiple breath loading) entails measuring the breath-by-breath alterations in breathing pattern after a step change in the external mechanics while CO2 is allowed to change or is controlled in the alveolar gas by computer control over inspired CO2. The second type of loading (intrabreath) will assess the neural control of a given breath when a mechanical load is applied at various times delayed from the start of inspiration. The responses to multiple breath loading will be analyzed by determining halftimes and steady-state magnitudes for the responses of ventilatory parameters. These responses will be compared to predicted optimal responses using workrate and mean force criteria and measured mechanical values for each subject. The intrabreath loading responses will be analyzed to determine changes in the effectiveness of a load to produce a change in cycle amplitude and phase duration as the loading point varies through the breathing cycle.